1. Field of the Invention
The present invention relates to a conductor substrate and a method of producing the same. More specifically, the invention relates to a conductor substrate useful for the production of a resin-sealed semiconductor device in which, after a semiconductor element is mounted, a portion mounting the element is sealed or covered with an insulating resin, and a method of producing the same. The invention is further concerned with a semiconductor device equipped with the above conductor substrate and a method of producing the same.
2. Description of Related Art
As is well known, there have been proposed a variety of semiconductor devices comprising a semiconductor element such as an IC chip or an LSI chip mounted on a substrate, and one of them is a resin-sealed semiconductor device in which a semiconductor element is mounted, and a portion mounting the semiconductor element is sealed with an insulating resin. The resin-sealed semiconductor device has a semiconductor element mounted on a conductor substrate such as a lead frame, along with necessary electric connection, and is often called a “semiconductor package”. The semiconductor packages are generally stored by the manufacturers after they are manufactured, and are offered to end users, depending upon their demand. Then, the end users mount the semiconductor packages on the substrates such as wiring boards by, for example, reflowing a solder to complete the final electronic equipment.
In the prior art process for producing electronic equipment, some problems have been found. One of the problems is caused during storage of the semiconductor devices. This is because, the resin sealing the semiconductor device tends to absorb moisture in the air while the semiconductor device is being stored until it is mounted on the substrate. The moisture absorbed by the sealing resin is rapidly vaporized and expanded due to the heat in the solder reflowing step (because a temperature of as high as about 180 to 200° C. is applied in this step) at the packaging the semiconductor device, producing a large stress in the sealing resin itself. As a result, cracks occur in the interface between the conductor substrate or the semiconductor element and the sealing resin, or the sealing resin peels off the conductor substrate. Such defects deteriorate the reliability of the semiconductor devices. It is therefore desired to provide a conductor substrate that is capable of sufficiently withstanding the stress due to vaporization and expansion of the moisture intimately adhering to the sealing resin or, in other words to provide a semiconductor device that can be stably stored for an extended period of time maintaining excellent adhesion.
Further, in recent years, from the standpoint of protecting global environment, there has been employed a lead-free solder instead of the conventional lead-containing solder in the step of solder reflowing in packaging the semiconductor device on the substrate. However, the lead-free solder has a melting point higher than that of the conventional solders (the conventional solders have a melting point of about 200° C., while the lead-free solder has a melting point of about 240 to 260° C.). Inevitably, therefore, the solder reflowing step must be conducted at a high temperature. As the packaging temperature increases, however, a larger stress is exerted on the semiconductor device, developing defects such as cracks in increased amounts.
A further study has been carried out in an attempt to solve the above-mentioned problems. For example, there has been proposed a method of forming a black oxide layer on a lead frame as illustrated in FIG. 1 to improve the adhesion of the sealing resin to the lead frame relying upon the anchoring effect (see, Japanese Unexamined Patent Publication (Kokai) No. 9-148509). The lead frame 101 illustrated is a press-molded article of copper or a copper alloy, and comprises a chip-mounting portion 102, an internal lead portion 103, an external lead portion 104, and a wire-bonding portion 105. Silver layer 102a and 105a are plated on the upper surfaces of the chip-mounting portion 102 and of the wire-bonding portion 105. Further, a circuit chip 106 is mounted on the chip-mounting portion 102. The circuit chip 106 and the wire-bonding portion 105 are connected together through a wire 107. Further, the lead frame 101 as a whole is sealed with a sealing resin 108. In order to improve the adhesion between the lead frame 101 and the sealing resin 108 relying upon the anchoring effect, a black oxide layer (cupric oxide CuO layer) 109 is formed on the limited portions where the silver layers 102a and 105a have not been plated. The black oxide layer 109 is formed upon anodization of the lead frame 101 in an organic alkali solution.
Further, referring to FIG. 2, there has been proposed a method of interposing a layer of cuprous oxide (Cu2O) between the conductor substrate and the blackened layer, i.e., black oxide layer (cupric oxide layer) so that the adhesion does not decrease between the conductor substrate and the sealing resin even when a high temperature is applied (see, Japanese Unexamined Patent Publication (Kokai) No. 2001-210776). Namely, the illustrated lead frame 113 is made of copper or a copper alloy, and has a cuprous oxide (Cu2O) layer 114, a cupric oxide (CuO) layer 112 and a molding resin layer 115 formed thereon in this order. The cuprous oxide layer 114 can be formed by forming the cupric oxide layer 112 by immersing the lead frame 113 in an alkali bath so as to be blackened and, then, heating the lead frame 113 at a high temperature in an oxidizing atmosphere.
According to these prior art methods, however, there still remains room for improvement. With the method of forming a black oxide layer on the surface of the conductor substrate by the anodization, for example, the resulting black oxide layer becomes too thick and, besides, an extended period of time is required for forming the layer. With the method of interposing a cuprous oxide layer between the conductor substrate and the cupric oxide layer to cope with the reflowing step that uses the lead-free solder, on the other hand, the production step becomes complex and a considerably extended period of time is required until the production is finished.